Tub drainage issue

[Serg]

When I accumulate a significant amount of water in my tub and then release the drain, it drains normally for a fair period of time. Then it slows down (or stops) for several seconds and then starts draining normally again. This repeats itself several times until all the water is drained out. My home warranty company sent a plumber out who said that the problem is that the drain goes down to a P-trap and, when it comes out of the P-trap, the drain line goes slightly upward until it dumps into the main sewer line. The plumber explains that the line probably goes slightly upward because it used to have a drum trap that was removed. This explanation does not seem right to me for the following reasons:

1. The point where the tub begins draining (i.e. the drain plug at the base of the tub) is at a point higher than the point where the drain line ultimately dumps into the main sewer line. Therefore, the overall flow is downward even if the last part is slightly upward.

2. If the problem were the slightly upward flow at the end, it would seem to me that the tub would drain consistently slowly, not drain at full speed, stop up for a few seconds and then drain at full speed again.

What do you all think? If you disagree with the plumber (who, by the way, is incentivized to blame it on the installation because it gets the home warranty company off the hook), do you have any other ideas of what could cause this type of symptom? Does it sound like a venting issue?

[tbplumbloco]

It could be what the plumber said if a drum trap were removed and a p-trap installed,the discharge from the drum trap has an established elevation that could make it difficult to repair with p-trap and get proper fall.Put a level on the section of pipe where the trap connects the horizontal pipe to the larger soil pipe.

[Serg]

There is no question that the last part (from the P-trap to the main sewer line) slopes slightly upward. I don't dispute that. My question was whether that is what is causing my problems. For the two reasons in my original post, I was not certain that was really causing my problems. Does it seem like the right cause given the symptoms I describe?

[tbplumbloco]

Is the tub vented?maybe when the drum-trap was removed they took a short cut and did not re-vent the tub.Another thing is the drain could need snaked, a test would be to turn the hot+cold water on the tub full blast,does the water drain or does it build up to the point that you would be standing in 3"to 6"of water,if it is backing up you have a drain problem,if it is draining, fine,but you may have a stoppage down the road because the pipe does not have proper fall.

[Serg]

When I run water at full blast, it more or less drains. There is a little build-up but not enough to even fill the bottom of the tub (less than 2" at its deepest by the drain and does not reach back of the tub). I think that is fairly normal. I only experience the problem when I actually plug and fill a good bit of the tub. When I do that, it drains normally, then stops up for a few seconds, gurgles a little, and then starts draining normally again. This process repeats itself until it fully drains. Im guessing a vent problem. The tub plugs into the original drain line, so the original vent should work, but maybe its a little stopped up. I will have to get on the roof and check. What do you think?

[tbplumbloco]

If it drains and does not leak forget about it.